Fiber-reinforced phthalonitrile composite cured with low-reactivity aromatic amine curing agent

ABSTRACT

A fiber-reinforced phthalonitrile composite is made by impregnating or coating a fibrous material with a phthalonitrile prepolymer mixture containing a phthalonitrile monomer and an aromatic amine curing agent that is thermally stable and nonvolatile at a temperature up to about 375° C., and that contains at least one electron withdrawing substituent effective to reduce the reactivity of the aromatic amine curing agent with the phthalonitrile monomer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the fabrication of fiber-reinforcedphthalonitrile composites and, in particular, to fiber-reinforcedphthalonitrile composites made by a method wherein the curing agent isselected to have low reactivity with the phthalonitrile monomer.

2. Description of the Related Art

Phthalonitrile resins made using amine curing agents are described inKeller, T. M. and Price, T. R., "Amine-Cured Bisphenol-LinkedPhthalonitrile Resins", J. Macromol. Sci.-Chem., A18(6), pp. 931-937(1982), U.S. Pat. No. 4,408,035 to Keller, U.S. Pat. No. 5,003,039 toKeller, U.S. Pat. No. 5,003,078 to Keller, U.S. Pat. No. 5,004,801 toKeller, U.S. Pat. No. 5,132,396 to Keller, U.S. Pat. No. 5,139,054 toKeller, U.S. Pat. No. 5,208,318 to Keller, U.S. Pat. No. 5,237,045 toBurchill et al, U.S. Pat. No. 5,292,854 to Keller and U.S. Pat. No.5,350,828 to Keller et al., the disclosures of which are incorporatedherein by reference.

Fiber-reinforced composites are typically made by heating aphthalonitrile monomer to its melt stage, adding a curing agent to themelted monomer to form a prepolymer mixture and then impregnating orcoating a fibrous material such as carbon fiber with the meltedprepolymer mixture. The fiber-containing prepolymer mixture is thenallowed to cure at an elevated temperature to form the fiber-reinforcedcomposite.

In creating fiber-reinforced composites by the method described above,it is necessary that the phthalonitrile prepolymer melt and flow easilyto completely adhere to and impregnate or coat the fibrous material. Aproblem that often arises is that the high temperature necessary formelting the phthalonitrile monomer also speeds the curing reaction,particularly if a fast-reacting amine such as1,3-bis(3-aminophenoxy)benzene is used as the curing agent. If thecuring reaction proceeds too rapidly, the increase in the viscosity ofthe prepolymer associated with the curing prevents the prepolymer fromflowing freely and completely permeating and impregnating or coating thefibrous material, resulting in a defective or resin-poor composite. Thisproblem may be overcome by using less of the amine curing agent (asshown, for example, in Sastri et al, "Phthalonitrile-Carbon FiberComposites" Polymer Composites, December 1996, Vol. 17, No.6, pp 816-822and Sastri et al "Phthalonitrile-Glass Fabric Composites", PolymerComposites, February 1997, Vol. 18, No. 1, pp 48-54, the disclosures ofwhich are incorporated herein by reference). However, using too littleof the curing agent results in insufficient and incomplete curing of thephthalonitrile resin. With a fast-reacting amine curing agent such as1,3-bis(3-aminophenoxy)benzene, the processing window between too muchcuring agent and not enough curing agent may be narrow. A narrowprocessing window can increase processing costs because greater caremust be taken to insure that the right amount of curing agent is used.

Another problem that arises in the fabrication of fiber-reinforcedphthalonitrile composites is that the high temperatures used in manymodern methods of composite fabrication tend to be above thedecomposition or volatilization temperature of many of the amine curingagents that have been traditionally used in the formation ofphthalonitrile polymers. Thus, there is a great need for amine curingagents that are thermally and oxidatively stable at very hightemperatures (up to about 375° C.).

An alternative method for creating a fiber-reinforced composite bypenetrating and impregnating or coating a fibrous material with aphthalonitrile prepolymer is to dissolve the phthalonitrile prepolymerin a solvent and then impregnate or coat a fibrous material with thesolvent/prepolymer mixture. However, the solvent method has its own setof problems such as the potential problem of creating undesirable voidsin the composite by the action of entrapped volatilized solventmolecules. Moreover, the solvent method involves additional elaborateprocessing steps and problems relating to removal and disposal of thesolvent.

In methods of creating fiber-reinforced composites by prepregconsolidation and filament winding, a long fibrous tow or filament isrun through or dipped into a container of a prepolymer melt orprepolymer/solvent mixture so that the tow or filament becomesthoroughly coated with the prepolymer, forming, for example, a prepregtape or a preform. In these methods of creating fiber-reinforcedcomposites, it is crucial that the prepolymer not cure too quicklybefore the coating process is complete.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide afiber-reinforced phthalonitrile composite made by impregnating orcoating a fibrous material with a phthalonitrile prepolymer in the meltstage and curing the prepolymer wherein the increase in viscosityassociated with the curing of the phthalonitrile prepolymer is delayedto allow thorough impregnation and penetration of the fibrous material.

It is a further object of the invention to provide a fiber-reinforcedphthalonitrile composite made by a process that includes the use of acuring additive wherein there is a large processing window for theamount of curing additive used.

It is a further object of the invention to provide a fiber-reinforcedphthalonitrile composite that is free of voids.

It is a further object of the invention to provide for the fabricationof fiber-reinforced phthalonitrile composites by the method of prepregconsolidation and filament winding whereby the phthalonitrile prepolymeris cured with a low-reactivity amine curing agent so that the prepolymerdoes not cure too quickly before the coating process is complete.

It is a further object of the invention to provide a fiber-reinforcedcomposite that is thermally and oxidatively stable at temperatures up toabout 375° C.

These and other objects are accomplished by providing a fiber-reinforcedthermoset composite made by a process comprising the steps of:

(a) heating a phthalonitrile monomer to its melt stage,

(b) combining the phthalonitrile monomer in the melt stage with anaromatic amine curing agent to form a prepolymer mixture

(c) heating the prepolymer mixture at a temperature greater than themelting temperature of the prepolymer mixture and equal to or less thanabout 375° C.

(d) impregnating or coating a fibrous material with the prepolymermixture to form a fiber-containing composition, and

(e) continuing to heat the fiber-containing composition at a temperatureabove the melting point of the prepolymer mixture and at or below about375° C. for a sufficient time to cure the fiber-containing compositionto form a fiber-reinforced composite, wherein the curing ischaracterized by an increase in viscosity of the fiber-containingcomposition and by gelation of the fiber-containing composition

wherein the aromatic amine curing agent is selected to have the propertyof being thermally stable and nonvolatile at a temperature up to about375° C.,

wherein the aromatic amine curing agent is added to the phthalonitrilemonomer in step (b) in an effective amount to completely cure thefiber-containing composition, and

wherein the aromatic amine curing agent contains at least one electronwithdrawing substituent effective to reduce the reactivity of thearomatic amine curing agent with the phthalonitrile monomer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a superimposed plot of viscosity vs. time for the curing of4,4'-bis(3,4-dicyanophenoxy)biphenyl prepolymer at 260° C. with thefollowing aromatic amine curing agents: 1,3-bis(3-aminophenoxy)benzene(m-APB) (comparative example), 1,4-bis(4-aminophenoxy)benzene (p-APB)(comparative example),bis[4-(4-aminophenoxy)phenyl]2,2'-hexafluoropropane (FA) (included in asecond patent application filed herewith by the same inventors),bis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS) and bis[4-(3-aminophenoxy)phenyl]sulfone (m-BAPS).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention relates to a fiber-reinforced thermoset composite made bya process of heating a phthalonitrile monomer to its melt stage,combining the phthalonitrile monomer in the melt stage with an aromaticamine curing agent to form a prepolymer mixture, heating the prepolymermixture at a temperature greater than the melting temperature of theprepolymer mixture and equal to or less than about 375° C., impregnatingor coating a fibrous material with the prepolymer mixture to form afiber-containing composition, and continuing to heat thefiber-containing composition at a temperature above the melting point ofthe prepolymer mixture and at or below about 375° C. for a sufficienttime to cure the fiber-containing composition to form a fiber-reinforcedcomposite, wherein the curing is characterized by an increase inviscosity of the fiber-containing composition and finally by a gelationof the fiber-containing composition.

The amine curing agent is selected to overcome certain problems specificto the creation of fiber-reinforced composites. In particular, thearomatic amine curing agent is selected to have the property of beingthermally stable and nonvolatile at temperatures above the melting pointof the phthalonitrile resin and up to about 375° C., so that it does notvolatilize or decompose and thereby cause voids in the finishedcomposite. The aromatic amine curing agent is further selected tocontain at least one electron withdrawing substituent effective toreduce the reactivity of the aromatic amine curing agent with thephthalonitrile monomer so that the fiber-containing prepolymer mixturedoes not completely cure until the fibrous material has been completelyimpregnated or coated. This property of reduced reactivity is especiallyuseful for making thick or multilayer composites.

Preferably, the aromatic amine curing agent is selected from the groupconsisting of

3,3'-dimethyl-4,4'-diaminodiphenylsulfone,

3,3'-diethoxy-4,4'-diaminodiphenylsulfone,

3,3'-dicarboxy-4,4'-diaminodiphenylsulfone,

3,3'-dihydroxy-4,4'-diaminodiphenylsulfone,

3,3'-disulfo-4,4'-diaminodiphenylsulfone,

3,3'-diaminobenzophenone,

4,4'-diaminobenzophenone,

3,3'-dimethyl-4,4'-diaminobenzophenone,

3,3'-dimethoxy-4,4'-diaminobenzophenone,

3,3'-dicarboxy-4,4'-diaminobenzophenone,

3,3'-dihydroxy-4,4'-diaminobenzophenone,

3,3'-disulfo-4,4'-diaminobenzophenone,

4,4'-diaminodiphenyl ethyl phosphine oxide,

4,4'-diaminodiphenyl phenyl phosphine oxide,

bis[4-(4-aminophenoxy)phenyl]sulfone,

bis[4-(3-aminophenoxy) phenyl]sulfone, and

bis(3-aminophenoxy-4'-phenyl)phenyl phosphine oxide.

Most preferred are aromatic amine curing agents of the formula:

    NH.sub.2 --Ar.sup.2 --O--Ar.sup.1 --X--Ar.sup.1 --O--Ar.sup.2 --NH.sub.2

where Ar¹ and Ar² are substituted or unsubstituted aromatic groups and Xis a electron withdrawing substituent selected from the group consistingof --CO--, --SO₂ --,--O--PO(R¹)--O-- and --PO(R¹)--, where R¹ is analkyl or aryl group, and aromatic amine curing agents of the formula

    NH.sub.2 --Ar.sup.4 --O--Ar.sup.3 --Z--Ar.sup.3 --O--Ar.sup.4 --NH.sub.2

wherein Z is a linking group or a connecting bond and Ar³ and Ar⁴ arearomatic groups and wherein either Ar³ or Ar⁴ or both Ar³ and Ar⁴ aresubstituted with at least one electron withdrawing substituent selectedfrom the group consisting of SO₂ R³, COOR⁴, OR⁵, COR⁶, SR⁷, C.tbd.CR⁸,Ar, and CH═C(R⁹)₂ where R³, R⁴, R⁵, R⁶, R⁷, R⁸ and R⁹ are hydrogen, analkyl group or an aryl group.

Any polymerizable phthalonitrile monomer may be used as the startingmonomer. Examples of suitable phthalonitrile monomers are given in U.S.Pat. No. 3,730,946, U.S. Pat. No. 3,763,210, U.S. Pat. No. 3,787,475,U.S. Pat. No. 3,869,499, U.S. Pat. No. 3,972,902, U.S. Pat. No.4,209,458, U.S. Pat. No. 4,223,123, U.S. Pat. No. 4,226,801, U.S. Pat.No. 4,234,712, U.S. Pat. No. 4,238,601, U.S. Pat. No. 4,304,896, U.S.Pat. No. 4,315,093, U.S. Pat. No. 4,351,776, U.S. Pat. No. 4,408,035,U.S. Pat. No. 4,409,782, U.S. Pat. No. 5,003,039, U.S. Pat. No.5,003,078, U.S. Pat. No. 5,159,054, U.S. Pat. No. 5,242,755, U.S. Pat.No. 5,352,760, and U.S. Pat. No. 5,464,926. All of these patents areincorporated herein by reference. For example, the phthalonitrilemonomer may be a monomer such as is described in U.S. Pat. No. 5,003,078and having the formula: ##STR1## where R is a tetravalent radical orsubstituted aromatic tetravalent radical of the general formula:##STR2## where X is ##STR3## any alkylene of up to six carbon atoms orany halogenated alkylene of up to six carbon atoms. By the word"substituted", it is meant that any known substituent could be attachedto the aromatic moiety. Substituents include but are not limited tohalogens, chalcogens, and organic radicals such as phenyl, alcohol,carboxyl, carbonyl, or aliphatic groups of less than 10 carbon atoms.The phthalonitrile monomer could also be a monomer such as is describedin U.S. Pat. No. 5,464,926 of the formula: ##STR4## wherein Arrepresents an aromatic group, R represents ##STR5## R' represents##STR6## y is an integer having a value of 0 to 4; and n represents anaverage value of from 1 to about 100.

Preferably, the phthalonitrile monomer is selected from the groupconsisting of 4,4'-bis(3,4-dicyanophenoxy)biphenyl,2,2-bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropane,2,2-bis[4-(3,4-dicyanophenoxy)phenyl]propane andbis[4-(3,4-dicyanophenoxy)phenyl]sulfone, or is a fluorine-containing ornon-fluorine-containing oligomeric multiple aromatic etherphthalonitrile monomer prepared from 4,4'-difluorobenzophenone,bisphenol A6F or a non-fluorinated bis-phenol, and 4-nitrophthalonitrileas described in U.S. Pat. No. 5,464,926.

The present invention allows for the use of phthalonitrile monomershaving high melting points or high curing temperatures, because thecuring reaction at high temperatures is slowed by the use of the lessreactive amines, and because the areomatic amine curing agent isselected to be thermally stable and nonvolatile at temperatures up toabout 375° C. as described above.

In preparing the fiber-reinforced composite according to the presentinvention, the phthalonitrile monomer is heated to a temperature aboveits melting temperature and the aromatic amine curing agent is added tothe melt. Some curing begins to take place as soon as the curing agentis added. The mixture can be used immediately to create afiber-reinforced composite or it can be quenched to form a B-stageprepolymer that can be stored indefinitely at room temperature and usedat a later time to create the fiber-reinforced composite.

The use of less reactive aromatic amine curing agents in accordance withthe present invention allows a greater ratio of the amine curing agentto the phthalonitrile monomer and allows for a greater processing windowbefore gelation occurs. Preferably, the amine curing agent is added inthe amount of 0.1-10 millimole %.

Any fibrous material suitable for forming fiber-reinforced compositescan be used in the present invention. Typical fibrous material includescarbon fibers, aramid fibers, glass fibers or ceramic fibers. Thefibrous material may be in any form including woven fabrics, nonwovenmats, or tow.

The steps of impregnating or coating the fibrous material to create afiber-containing composition and of curing the fiber-containingcomposition to form a fiber-reinforced composite may by carried out byany method known in the art for creating fiber-reinforced composites. Inparticular, conventional methods of prepreg consolidation, filamentwinding, resin transfer and resin infusion may be used. For example,Sastri et al "Phthalonitrile-Glass Fabric Composites", PolymerComposites, February 1997, Vol. 18, No. 1, page 51 describes resininfusion as follows: "The resin infusion process has been reported byAhn and coworkers . . . as an alternative to the conventional laminationprocess using prepregs. This technique simplifies the compositemanufacturing process where impregnation and consolidation are combinedin a single step. In this process, unlike with prepregs, each ply ismade up of dry fibers while the matrix resin is placed at the top orbottom. A mold is formed by the sealant tape and bagging materials andthe matrix resin may be in the form of a film, powder, pellet or viscousliquid. This processing technique is particularly attractive for resinsthat can exhibit low viscosity values at elevated temperatures." As usedherein, the term "impregnating" a fibrous material means saturating thefibrous material with the prepolymer mixture, such as is typically donein the conventional methods of resin transfer and resin infusion orother methods. The term "coating" the fibrous material means coveringthe fibrous material with the prepolymer mixture such as is typicallydone in conventional methods of prepreg consolidation and filamentwinding or other methods.

Having described the invention, the following examples are given toillustrate specific applications of the invention, including the bestmode now known to perform the invention. These specific examples are notintended to limit the scope of the invention described in thisapplication.

EXAMPLES Example 1 (Comparative Example)4,4'-Bis(3,4-dicyanophenoxy)biphenyl prepolymer synthesis with1,3-bis(3-aminophenoxy)benzene (m-APB) (Composition 1)

100 g of 4,4'-bis(3,4-dicyanophenoxy)biphenyl monomer was melted in a500 ml reaction kettle equipped with a mechanical stirrer. The initialtemperature was maintained at about 280° C. and subsequent to monomermelting, the temperature was lowered to 255° C. At this time, 2.50 wt. %of 1,3-bis(3-aminophenoxy)benzene (m-APB) (8.55 mmoles) was added to themonomer melt, stirred for 15 minutes and quenched to room temperature.The prepolymer was pulverized to a fine powder and used for curestudies.

Example 2 (Comparative Example) 4,4'-Bis(3,4-dicyanophenoxy)biphenylprepolymer synthesis with 1,3-bis(3-aminophenoxy)benzene (m-APB)(Composition 2)

100 g of 4,4'-bis(3,4-dicyanophenoxy)biphenyl monomer was melted in a500 ml reaction kettle equipped with a mechanical stirrer. The initialtemperature was maintained at about 280° C.; and subsequent to monomermelting, the temperature was lowered to 255° C. At this time, 3.97 wt. %of 1,3-bis(3-aminophenoxy)benzene (m-APB) (13.58 mmoles) was added tothe monomer melt, stirred for 15 minutes and quenched to roomtemperature. The prepolymer was pulverized to a fine powder and used forcure studies.

Example 3 (Comparative Example) 4,4'-Bis(3,4-dicyanophenoxy)biphenylprepolymer synthesis with 1,4-bis(4-aminophenoxy)benzene (p-APB)

100 g of 4,4'-bis(3,4-dicyanophenoxy)biphenyl monomer was melted in a500 ml reaction kettle equipped with a mechanical stirrer. The initialtemperature was maintained at about 280° C.; and subsequent to monomermelting, the temperature was lowered to 255° C. At this time, 2.50 wt. %of 1,4-bis(4-aminophenoxy)benzene (p-APB) (8.55 mmoles) was added to themonomer melt, stirred for 15 minutes and quenched to room temperature.The prepolymer was pulverized to a fine powder and used for curestudies.

Example 4 4,4'-Bis(3,4-dicyanophenoxy)biphenyl prepolymer synthesis withbis[4-(4-aminophenoxy)phenyl]2,2'-hexafluoropropane (FA)

100 g of 4,4'-bis(3,4-dicyanophenoxy)biphenyl monomer was melted in a500 ml reaction kettle equipped with a mechanical stirrer. The initialtemperature was maintained at about 280° C.; and subsequent to monomermelting, the temperature was lowered to 255° C. At this time, 4.43 wt. %of bis[4-(4-aminophenoxy)phenyl]2,2'-hexafluoropropane (FA) (8.55mmoles) was added to the monomer melt, stirred for 15 minutes andquenched to room temperature. The prepolymer was pulverized to a finepowder and used for cure studies.

Example 5 4,4'-Bis(3,4-dicyanophenoxy)biphenyl prepolymer synthesis withbis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS) (Composition 1')

100 g of 4,4'-bis(3,4-dicyanophenoxy)biphenyl monomer was melted in a500 ml reaction kettle equipped with a mechanical stirrer. The initialtemperature was maintained at about 280° C.; and subsequent to monomermelting, the temperature was lowered to 255° C. At this time, 2.5 wt. %of bis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS) (5.79 mmoles) was addedto the monomer melt, stirred for 15 minutes and quenched to roomtemperature. The prepolymer was finely ground to a fine powder and usedfor cure studies.

Example 6 4,4'-bis(3,4-dicyanophenoxy)biphenyl prepolymer synthesis withbis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS) (Composition 2')

100 g of 4,4'-bis(3,4-dicyanophenoxy)biphenyl monomer was melted in a500 ml reaction kettle equipped with a mechanical stirrer. The initialtemperature was maintained at about 280° C.; and subsequent to monomermelting, the temperature was lowered to 255° C. 3.69 wt. % ofbis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS) (8.55 mmoles) was added tothe monomer melt, stirred for 15 minutes and quenched to roomtemperature. The prepolymer was finely ground to a fine powder and usedfor cure studies.

Example 7 4,4'-Bis(3,4-dicyanophenoxy)biphenyl prepolymer synthesis withbis[4-(3-aminophenoxy) phenyl]sulfone (m-BAPS)

100 g of 4,4'-bis(3,4-dicyanophenoxy)biphenyl monomer was melted in a500 ml reaction kettle equipped with a mechanical stirrer. The initialtemperature was maintained at about 280° C.; and subsequent to monomermelting, the temperature was lowered to 250° C. At this time, 3.69 wt. %of bis[4-(3-aminophenoxy) phenyl]sulfone (m-BAPS) (8.55 mmoles) wasadded to the monomer melt, stirred for 15 min. and quenched to roomtemperature. The prepolymer was pulverized to a fine powder and used forcure studies.

Example 8 2,2-Bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropaneprepolymer synthesis with bis[4-(4-aminophenoxy) phenyl]sulfone (p-BAPS)

1.5 g of 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropane monomerwas melted in an aluminum planchet on top of a hot plate. To the melt at250° C. was added 3.0 wt. % of bis[4-(4-aminophenoxy) phenyl]sulfone(p-BAPS) (0.104 mmoles) with stirring followed by quenching to roomtemperature after 15 minutes. The prepolymer was finely ground to a finepowder and used for cure studies.

Example 9 2,2-Bis[4-(3,4-dicyanophenoxy)phenyl]propane prepolymersynthesis with bis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS)

1.5 g of 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]propane monomer was meltedin an aluminum planchet on top of a hot plate. To the melt at 250° C.was added 2.0 wt. % of bis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS)(0.069 mmoles) with stirring followed by quenching to room temperatureafter 15 minutes. The prepolymer was finely ground to a fine powder andused for cure studies.

Example 10 Bis[4-(3,4-dicyanophenoxy)phenyl]sulfone prepolymer synthesiswith 4,4'-diaminobenzophenone

1.5 g of bis[4-(3,4-dicyanophenoxy)phenyl]sulfone monomer was melted inan aluminum planchet on top of a hot plate. To the melt at 250° C. wasadded 2.0 wt. % of 4,4'-diaminobenzophenone (0.150 mmoles) with stirringfollowed by quenching to room temperature after 15 minutes. Theprepolymer was finely ground to a fine powder and used for cure studies.

Example 11 Oligomeric multiple aromatic ether-containing phthalonitrilemonomer prepolymer synthesis with bis[4-(4-aminophenoxy)phenyl]sulfone(p-BAPS)

1.5 g of oligomeric multiple aromatic ether-containing phthalonitrilemonomer prepared from 4,4'-difluorobenzophenone (12.01 g, 55.1 mmol),4,4'-dihydroxybiphenyl (20.11 g, 108.1 mmol), and 4-nitrophthalonitrile(19.0 g, 109.8 mmol) was melted in an aluminum planchet on top of a hotplate. To the melt at 250° C. was added 2.0 wt. % ofbis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS) (0.069 mmoles) withstirring followed by quenching to room temperature after 15 minutes. Theprepolymer was finely ground to a fine powder and used for cure studies.

Example 12 Phthalonitrile cure characterization by differential scanningcalorimetry (DSC) and thermogravimetric analysis (TGA)

DSC studies were performed on mixturesof4,4'-bis(3,4-dicyanophenoxy)biphenyl monomer and amines (used inExamples 1-7) to monitor the cure reaction exotherm. Typically, 5-8 mgof monomer and 2-5 wt. % amine were weighed in a Perkin Elmer DSC pan,sealed and heated at 10° C./min. With all the monomer/amine mixturesamples, i. e. with amines that contain electron donating groups on thearomatic ring (m-APB and p-APB) as well as the ones with electronwithdrawing groups (such as in FA, p-BAPS and m-BAPS), the DSC tracesshowed the amine melting peak (an endotherm), monomer melting peakaround 250° C. (an endotherm) and a small exothermic peak between 250and 260° C. corresponding to the reaction of the amine with the monomer.Thus, it appears that the cure exotherm position is not very sensitiveto the presence of either electron donating or electron withdrawinggroups on the reacting amines.

Approximately, 1 g of the each prepolymer from Examples 1-7 was placedin an aluminum planchet and subjected to a heat treatment of 16 hours at250° C. in an air circulating oven. A vitrified product results uponheat treatment in all cases. The thermal stability of the variouspolymers was ascertained by TGA studies on powdered samples under aninert atmosphere of nitrogen. Results indicate that in all cases, thepolymer is stable up to 400° C. and begins to lose weight thereafter.When the powdered samples are postcured to elevated temperatures of 8hours at 350° C. and 8 hours at 375° C. in the TGA furnace, all samplesshow improved thermal stability with weight loss occurring only above500° C. The samples also retain about 65-70% char upon pyrolysis to1000° C. under inert conditions. In an oxidative environment, samplescured to 250° C. show rapid weight loss above 400° C., typical of mostcarbon-based materials. Samples that are postcured to elevatedtemperatures of 8 hours at 350° C. and 8 hours at 375° C. in the TGAfurnace show better thermo-oxidative stability than the 250° C. curedsamples and are stable up to 500° C. Thus, even in the thermal analysesmeasurements, the curing additives with electron donating groups (i. e.m-APB and p-APB) and the curing agents with electron withdrawing groups(FA, p-BAPS and m-BAPS) afford polymers with comparable thermal andoxidative stabilities.

Example 13 Cure studies on phthalonitrile prepolymers from Examples 1and 2 (Comparative Example)

1 g of each prepolymer made with amine contents 2.5% and 3.97% by wt.(composition 1 and composition 2 described in Examples 1 and 2,respectively) was placed in an aluminum planchet and heated on a hotplate at 250° C. The viscosity increased very rapidly in the sample witha higher amine content (3.97 wt. %) and a vitrified product resultedwithin 25-30 minutes of the reaction time. On the other hand, the samplewith 2.5% amine built up viscosity slower and a vitrified productresulted after about 5 hours of reaction time at 250° C. This experimentdemonstrates that the processability and curing rate of prepolymers thatare cured using an aromatic amine curing agent that has an electrondonating group on the aromatic ring is very sensitive to theconcentration of the curing agent. In particular, the prepolymer madewith 3.97% amine content using m-APB or p-APB as curing agents wouldcure too quickly to be useful in making a fiber-reinforced composite.

Example 14 Cure studies on phthalonitrile prepolymers from Examples 5and 6

1 g of each prepolymer made with p-BAPS amine content of 2.5% and 3.69%by wt. (composition 1' and composition 2' described in Examples 5 and 6,respectively) was placed in an aluminum planchet and heated on a hotplate at 250° C. The viscosity increase was faster with the composition2' relative to the prepolymer with composition 1'. However, contrary tothat described in Example 13, both samples required much longer dwellsat elevated temperatures for vitrified products to result. For instance,even after 16 hours at 250° C., composition 1' remained a viscous masswhereas composition 2' yielded a vitrified product after about 8 hoursat 250° C. These results suggest that the reactivity of p-BAPS amine islower compared to the m-APB amine and this difference may be attributedto the presence of an electron withdrawing group in the former amine andan electron donating group in the latter case. It may be inferred thatthe curing of prepolymers cured with aromatic amine curing agentscontaining electron withdrawing groups may be better controllable thanthose with electron donating moieties and that the slow-reacting amines,because they maintain a low viscosity for a longer period of time, aremore suitable for making fiber-reinforced composites.

Example 15 Rheometric studies on phthalonitrile prepolymers fromExamples 1-7

Viscosity studies were conducted using 1.5 g of powdered prepolymersamples and 40 mm parallel plate fixture. For comparative studies,prepolymers made with 8.55 mmoles of amine (i. e. samples from Examples1, 3, 4, 6 and 7) were used. Data collected at 260° C. (shown in FIG. 1)reveals that aromatic amine curing agents that contain electron donatinggroups on the aromatic rings (m-APB and p-APB) catalyze thephthalonitrile cure at a faster rate compared to those amines whichcontain electron withdrawing groups (FA, m-BAPS and p-BAPS). In additionto slowing down the cure, larger quantities of amines with electronwithdrawing groups could be used for prepolymer synthesis and thefabrication of fiber-reinforced composites in a more controlled manner.In other words, such amines broaden the processing window forfabricating phthalonitrile-based fiber-reinforced composites.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A method of preparing a fiber-reinforcedthermoset composite comprising the steps of:(a) heating a phthalonitrilemonomer to its melt stage, (b) combining the phthalonitrile monomer inthe melt stage with an aromatic amine curing agent to form a prepolymermixture, (c) heating the prepolymer mixture at a temperature greaterthan the melting temperature of the prepolymer mixture, (d) impregnatinga multilayer fibrous material with the heated prepolymer mixture to forma fiber-containing composition and heating the fiber-containingcomposition at a temperature above the melting point of the prepolymermixture for a sufficient time to cure the fiber-containing compositionto form a fiber-reinforced composite, the curing being characterized byan increase in viscosity and gelation of the prepolymer mixture in thefiber-containing composition, wherein the rate of curing of thefiber-containing composition is controlled by selecting an aromaticamine curing agent having at least one electron withdrawing substituentand having a reactivity with the phthalonitrile monomer that issufficiently low so that the increase in viscosity and gelation of theprepolymer mixture that occurs when the fiber-containing compositioncures is delayed until after the multilayer fibrous material isthoroughly impregnated, and wherein the aromatic amine curing agent isselected to have the property of being thermally stable and nonvolatileat a temperature up to about 375° C.
 2. The method of claim 1 whereinthe multilayer fibrous material is in the form of fiber braids.
 3. Themethod of claim 1 wherein the multilayer fibrous material is carbonfibers.
 4. The method of claim 1 wherein the multilayer fibrous materialis glass fibers.
 5. The method of claim 1 wherein the step (d) ofimpregnating the multilayer fibrous material with the heated prepolymermixture is carried out by resin transfer molding.
 6. The method of claim1 wherein the step (d) of impregnating the multilayer fibrous materialwith the heated prepolymer mixture is carried out by resin infusionmolding.
 7. The method of claim 1, wherein the aromatic amine curingagent has the general formula

    NH.sub.2 --Ar.sup.2 --O--Ar.sup.1 --X--Ar.sup.1 --O--Ar.sup.2 --NH.sub.2

where Ar¹ and Ar² are substituted or unsubstituted aromatic groups and Xis the electron withdrawing substituent.
 8. The method of claim 7,wherein X is selected from the group consisting of --CO--, --SO₂ --,--O-- PO(R¹)--O-- and --PO(R¹)--, where R¹ is an alkyl or aryl group. 9.The method of claim 8, wherein the aromatic amine curing agent has thegeneral formula:

    NH.sub.2 --Ar.sup.4 --O--Ar.sup.3 --Z--Ar.sup.3 --O--Ar.sup.4 --NH.sub.2

wherein Z is a linking group or a connecting bond and Ar³ and Ar⁴ arearomatic groups and wherein either Ar³ or Ar⁴ or both Ar³ and Ar⁴ aresubstituted with at least one electron withdrawing substituent.
 10. Themethod of claim 9 wherein the electron withdrawing substituent isselected from the group consisting of SO₂ R³, COOR⁴, OR⁵, COR⁶, SR⁷,C.tbd.CR⁸, Ar, and CH═C(R⁹)₂ where R³, R⁴, R⁵, R⁶, R⁷, R⁸ and R⁹ arehydrogen, an alkyl group or an aryl group.
 11. The method of claim 1,wherein the aromatic amine curing agent is selected from the groupconsisting of3,3'-dimethyl-4,4'-diaminodiphenylsulfone,3,3'-diethoxy-4,4'-diaminodiphenylsulfone,3,3'-dicarboxy-4,4'-diaminodiphenylsulfone,3,3'-dihydroxy-4,4'-diaminodiphenylsulfone,3,3'-disulfo-4,4'-diaminodiphenylsulfone, 3.3'-diaminobenzophenone,4,4'-diaminobenzophenone,3,3'-dimethyl-4,4'-diaminobenzophenone,3,3'-dimethoxy-4,4'-diaminobenzophenone,3,3'-dicarboxy-4,4'-diaminobenzophenone,3,3'-dihydroxy-4,4'-diaminobenzophenone,3,3'-disulfo-4,4'-diaminobenzophenone, 4,4'-diaminodiphenyl ethylphosphine oxide, 4,4'-diaminodiphenyl phenyl phosphine oxide,bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, and bis(3-aminophenoxy-4'-phenyl)phenyl phosphine oxide.12. The method of claim 1, wherein the phthalonitrile monomer is acompound of the formula: ##STR7## where R is a tetravalent radical orsubstituted aromatic tetravalent radical of the general formula:##STR8## where X is ##STR9## any alkylene of up to six carbon atoms orany halogenated alkylene of up to six carbon atoms.
 13. The method ofclaim 1, wherein the phthalonitrile monomer is a compound of theformula: ##STR10## wherein Ar represents an aromatic group, R represents##STR11## R' represents ##STR12## y is an integer having a value of 0 to4; and n represents an average value of from 1 to about
 100. 14. Themethod of claim 1 wherein the phthalonitrile monomer is selected fromthe group comprising 4,4'-bis(3,4-dicyanophenoxy)biphenyl,2,2-bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropane,2,2-bis[4-(3,4-dicyanophenoxy)phenyl]propane andbis[4-(3,4-dicyanophenoxy)phenyl]sulfone and the aromatic amine curingagent is bis[4-(4-aminophenoxy)phenyl]sulfone.
 15. The method of claim 1wherein the phthalonitrile monomer is selected from the group comprising4,4'-bis(3,4-dicyanophenoxy)biphenyl,2,2-bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropane,2,2-bis[4-(3,4-dicyanophenoxy)phenyl]propane andbis[4-(3,4-dicyanophenoxy)phenyl]sulfone and the aromatic amine curingagent is bis[4-(3-aminophenoxy)phenyl]sulfone.
 16. A method of preparinga fiber-reinforced thermoset composite comprising the steps of:(a)heating a phthalonitrile monomer to its melt stage, (b) combining thephthalonitrile monomer in the melt stage with an aromatic amine curingagent to form a prepolymer mixture, (c) heating the prepolymer mixtureat a temperature greater than the melting temperature of the prepolymermixture, (d) coating a fibrous material with the heated prepolymermixture by a filament winding process to form a fiber-containingcomposition and heating the fiber-containing composition at atemperature above the melting point of the prepolymer mixture for asufficient time to cure the fiber-containing composition to form afiber-reinforced composite, the curing being characterized by anincrease in viscosity and gelation of the prepolymer mixture in thefiber-containing composition, wherein the rate of curing of thefiber-containing composition is controlled by selecting an aromaticamine curing agent having at least one electron withdrawing substituentand having a reactivity with the phthalonitrile monomer that issufficiently low so that the increase in viscosity and gelation of theprepolymer mixture that occurs when the fiber-containing compositioncures is delayed until after the fibrous material is thoroughlyimpregnated and the filament winding process is complete, and whereinthe aromatic amine curing agent is selected to have the property ofbeing thermally stable and nonvolatile at a temperature up to about 375°C.